Why Do Some Cars Need Aerofoils?

Racing cars capable of moving at speeds well in excess of 100 mph need aerofoil’s to counter the effect of lift created by their highly streamlined shapes. Drag is caused by the turbulence of the air as it flows in to fill the vacuum left behind a forward moving object.

The less streamlined the object, the greater the space to be filled in and the greater the turbulence, therefore, the greater the “drag”. As designers improve the streamlining effect of racing cars to reduce drag, the cars’ shapes become more and more flattened and a new problem arises.

The car has taken on similar properties to an aircraft wing and the movement of air over and under it at high speed begins to provide lift. The aerofoil placed at the rear of the car is so designed as to create a downward force when it passes through air at speed, and thus counteracts the lift. If this did not occur the cars’ wheels would make too little contact with the road surface and the driver would quickly lose control.

What is an aerofoil and spoiler and what does it do? Well, an airplane wing pushes the plane up by using the Coinda effect and the Bernoulli paradox. Air keeps travelling in the same direction after deflection, fast moving air creates low pressure. When the air over the wing is going faster than that underneath it generates lift. In a car the principle is reversed causing down force.

The whole shape of a car is an aerodynamic nightmare. The cabin area acts like an airplane wing and forces the air to travel over it at great speed creating lift. When you are driving along lift is the last thing you want. The more securely planted on the ground the wheels are the better at the handling and traction.

Spoilers tend to diffuse the air flowing over the final edge of a car whereas aerofoils or wings are mounted higher up and produce down force, pushing the car into the road. The aerofoils are used to great effect in motor sports and can actually generate more down force than the car weighs in the first place. This would mean that a car at speed can travel on the roof of a tunnel! Although getting it up there in the first place might present a challenge.

A holistic approach to race car dynamics needs to be applied. The front spoiler needs to create a good fast flow of air under the car and this need to be coupled with diffusers at the rear. Side skirts will help keep this air flowing true and straight and will improve cornering speeds and grip.

Front splitters are further added to keep the nose of the car down. With an aerofoil on the rear the car can have a tendency to lift at the front, so overlook the front aerodynamics at your peril.

It should be noted that aerofoils and spoilers do not even start to have a noticeable effect until you hit speeds of 70mph. Up to this point they are a cause of drag and will rob you of fuel economy. Any car competing at speeds of 70mph or more should have it aerodynamic tuning fully sorted. It makes the car safer and will allow you to take the cars grip levels to unimaginable heights.

Get an adjustable aerofoil to allow you to set it up to give the optimum effect. A twin plane rear aerofoil works best and using carbon fibre will keep the weight down. Rigidity is important however and you need a wing that does not deform under pressure. Vertical slats can be used to aid straight line stability.

Content for this question contributed by Curtis Reinhardt, resident of Chino, San Bernardino County, California, USA